"Doris," she began, "this is my secret--and Genevieve's--and I never thought I'd tell any one about it. It's the only secret I ever had worth anything, but I'm going to tell you,--well, because I--I think so much of you. Will you solemnly promise--cross your heart--that you'll never tell any one?"

Doris gazed straight into Sally's somewhat troubled eyes. "I don't need to 'cross my heart,' Sally. I just give you my word of honor I won't, unless sometime you wish it. I've not breathed a word of the fact that you _had_ a secret, even to Mother. And I've never kept anything from her before." And this simple statement completely satisfied Sally.

"Come on, then," she said. "Follow Genevieve and me, and we'll give you the surprise of your life."

She grasped her small sister's hand and led the way, and Doris obediently followed. To her surprise, however, they did not scramble up the sandy pine-covered slope as usual, but picked their way, instead, along the tiny strip of beach on the farther side of the point where the river ate into the sh.o.r.e in a great, sweeping cove. After trudging along in this way for nearly a quarter of a mile, Sally suddenly struck up into the woods through a deep little ravine. It was a wild scramble through the dense underbrush and over the boughs of fallen pine trees.

Sally and Genevieve, more accustomed to the journey, managed to keep well ahead of Doris, who was scratching her hands freely and doing ruinous damage to her clothes plunging through the th.o.r.n.y tangle. At last the two, who were a distance of not more than fifty feet ahead of her, halted, and Sally called out:

"Now stand where you are, turn your back to us and count ten--slowly.

Don't turn round and look till you've finished counting." Doris obediently turned her back, and slowly and deliberately "counted ten."

Then she turned about again to face them.

To her complete amazement, there was not a trace of them to be seen!

Thinking they had merely slipped down and hidden in the undergrowth to tease her, she scrambled to the spot where they had stood. But they were not there. She had, moreover, heard no sound of their progress, no snapping, cracking or breaking of branches, no swish of trailing through the vines and high gra.s.s. They could not have advanced twenty feet in any direction, in the short time she had been looking away from them. Of both these facts she was certain. Yet they had disappeared as completely as if the earth had opened and swallowed them. Where, in the name of all mystery, could they be?

Doris stood and studied the situation for several minutes. But, as they were plainly nowhere in her vicinity, she presently concluded she must have been mistaken about their not having had time to get further away, and determined to hunt them up.

So away she pursued her difficult quest, becoming constantly more involved in the thick undergrowth and more scratched and dishevelled every moment, till at length she stood at the top of the bluff. From this point she could see in every direction, but not a vestige of Sally or Genevieve appeared. More bewildered than ever, Doris clambered back to the spot where she had last seen them. And, as there was plainly now no other course, she stood where she was and called aloud:

"Sally! Sal--ly! I give it up. Where in the world are you?"

There was a low, chuckling laugh directly behind her, and, whirling about, she beheld Sally's laughing face peeping out from an aperture in the tangled growth that she was positive she had not noticed there before.

"Come right in!" cried Sally. "And I won't keep it a secret any longer.

Did you guess it was anything like this?"

She pushed a portion of the undergrowth back a little farther and Doris scrambled in through the opening. No sooner was she within than Sally closed the opening with a swift motion and they were all suddenly plunged into inky darkness.

"Wait a moment," she commanded, "and I'll make a light." Doris heard her fumbling for something; then the scratch of a match and the flare of a candle. With an indrawn breath of wonder, Doris looked about her.

"Why, it's a room!" she gasped. "A little room all made right in the hillside. How did it ever come here? How did you ever find it?"

It was indeed the rude semblance of a room. About nine feet square and seven high, its walls, floor and ceiling were finished in rough planking of some kind of timber, now covered in the main with mold and fungus growths. Across one end was a low wooden structure evidently meant for a bed, with what had once been a hard straw mattress on it. There was likewise a rudely constructed chair and a small table on which were the rusted remains of a tin platter, knife and spoon. There was also a metal candle-stick in which was the candle recently lit by Sally. It was a strange, weird little scene in the dim candle-light, and for a time Doris could make nothing of its riddle.

"What _is_ it? What does it all mean, Sally?" she exclaimed, gazing about her with awestruck eyes.

"I don't know much more about it than you do," Sally averred. "But I've done some guessing!" she ended significantly.

"But how did you ever come to discover it?" cried Doris, off on another tack. "I could have searched Slipper Point for years and never have come across _this_."

"Well, it was just an accident," Sally admitted. "You see, Genevieve and I haven't much to do most of the time but roam around by ourselves, so we've managed to poke into most of the places along the sh.o.r.e, the whole length of this river, one time and another. It was last fall when we discovered this. We'd climbed down here one day, just poking around looking for beach-plums and things, and right about here I caught my foot in a vine and went down on my face plumb right into that lot of vines and things. I threw out my hands to catch myself, and instead of coming against the sand and dirt as I'd expected, something gave way, and when I looked there was nothing at all there but a hole.

"Of course, I poked away at it some more, and found that there was a layer of planking back of the sand. That seemed mighty odd, so I pushed the vines away and banged some more at the opening, and it suddenly gave way because the boards had got rotten, I guess, and--I found _this_!"

Doris sighed ecstatically. "What a perfectly glorious adventure! And what did you do then?"

"Well," went on Sally simply, "although I couldn't make very much out of what it all was, I decided that we'd keep it for our secret,--Genevieve and I--and we wouldn't let another soul know about it. So we pulled the vines and things over the opening the best we could, and we came up next day and brought some boards and a hammer and nails--and a candle. Then I fixed up the rotten boards of this opening,--you see it works like a door, only the outside is covered with vines and things so you'd never see it,--and I got an old padlock from Dad's boathouse and I screwed it on the outside so's I could lock it up besides, and covered the padlock with vines and sand. n.o.body'd ever dream there was such a place here, and I guess n.o.body ever has, either. That's my secret!"

"But, Sally," exclaimed Doris, "how did it ever come here to begin with?

Who made it? It must have some sort of history."

"There you've got _me_!" answered Sally.

"Some one must have stayed here," mused Doris, half to herself. "And, what's more, they must have _hidden_ here, or why should they have taken such trouble to keep it from being discovered?"

"Yes, they've hidden here, right enough," agreed Sally. "It's the best hiding place any one ever had, I should say. But the question is, what did they hide here for?"

"And also," added Doris, "if they were hiding, how could they make such a room as this, all finished with wooden walls, without being seen doing it? Where did they get the planks?"

"Do you know what that timber is?" asked Sally.

"Why, of course not," laughed Doris. "How should I?"

"Well, I do," said her companion. "I know something about lumber because Dad builds boats and he's shown me. I scratched the mold off one place,--here it is,--and I discovered that this planking is real seasoned cedar like they build the best boats of. And do you know where I think it was got? It came from some wrecked vessel down on the beach.

There are plenty of them cast up, off and on, and always have been."

"But gracious!" cried Doris, "how was it got here?"

"Don't ask me!" declared Sally. "The beach is miles away."

They stood for some moments in silence, each striving to piece together the story of this strange little retreat from the meagre facts they saw about them. At last Doris spoke.

"Sally," she asked, "was this all you ever found here? Was there absolutely nothing else?" Sally started, as if surprised at the question and hesitated a moment.

"No," she acknowledged finally. "There _was_ something else. I wasn't going to tell you right away, but I might as well now. I found this under the mattress of the bed."

She went over to the straw pallet, lifted it, searched a moment and, turning, placed something in Doris's hands.

CHAPTER V

MYSTERY

Doris received the object from Sally and stood looking at it as it lay in her hands. It was a small, square, very flat tin receptacle of some kind, rusted and moldy, and about six inches long and wide. Its thickness was probably not more than a quarter of an inch.

"What in the world is it?" she questioned wonderingly.

"Open it and see!" answered Sally. Doris pried it open with some difficulty. It contained only a sc.r.a.p of paper which fitted exactly into its s.p.a.ce. The paper was brown with age and stained beyond belief. But on its surface could be dimly discerned a strange and inexplicable design.

"Of _all_ things!" breathed Doris in an awestruck voice. "This certainly is a mystery, Sally. What _do_ you make of it?"

"I don't make anything of it," Sally averred. "That's just the trouble.

I can't imagine what it means. I've studied and studied over it all winter, and it doesn't seem to mean a single thing."

It was indeed a curious thing, this sc.r.a.p of stained, worn paper, hidden for who knew how many years in a tin box far underground. For the riddle on the paper was this: