590 STONE IMPLEMENTS FEOM SHORES OF LAKE MICHIGAN. 



under repeated blows that especially distinguishes this rock — a quality 

 that made it the favorite material for hammer stones throughout the 

 region. Tough, and merely battered by ordinary hammering, it is 

 still capable of flaking under heavy blows, and with a characteristic 

 effect when (pertain conditions of the mass worked are present, large 

 aud symmetrical flakes resulting. While the fracture is not as clean 

 cut as that produced in flaking flint and similar materials, it is never- 

 theless of the same general character.' 



TJie flaked cobblestones. — The mixed beach gravels east of the shaping 

 sites afford a supply of well-rounded cobblestones, those used averaging 

 perhaps 4 inches in diameter aud 1^ inches in thickness. The clay 

 bluffs west of the sites supply the same rock in angular masses, but 

 these are not reiDresented in the refuse under consideration, though 

 many were carried to the camp sites for a variety of uses. The amount 

 of refuse of flaking varies with different sites. In some places a single 

 piece only has been flaked, while again as many as twenty or thirty 

 must have been worked up. Occasionally a group of four or five well- 

 selected cobbles occurs in connection with small quantities of refuse, 

 as if a part only of the stock gathered had been worked up. The num- 

 ber of flakes removed from a single stone was never great. Many 

 blows were struck without producing flakes, the point subjected to 

 percussion exhibiting only a gray mark or depression where the sur- 

 face crumbled under the shock. Such marks commonly occur singly 

 or in groups at the usual points of impact, but in cases extend quite 

 around the stone. 



A successful blow, one producing the desired flake, is represented by 

 a notch in the margin of the resultant facet from which slight furrows 

 and ridges radiate for a short distance over the face of fracture. (See 

 Plate I.) The notch is often light gray or even white in fresh-looking 

 specimens ; these percussion marks usually occur singly, but instances of 

 two or even three notches in one facet are met with. The facet is com- 

 monly circular or oval in outline and often of considerable size, reach- 

 ing in cases 4 or 5 inches in diameter; many, as a matter of course, 

 are small and some are irregular in contour. A single facet sometimes 



1 1 am greatly iudebted to Mr. W. J. Stebbins for the exact identification of this 

 rock. A number of flakes submitted to him for analysis gave the following result: 

 " Under the microscope, sections of each of the rock specimens submitted were found 

 to be diabase, in each case composed of oligoclase, augite, chlorite, calcite, and iron. 

 That they have been subjected to much weathering and wearing from water action 

 is evident from their outward appearance. The most noticeable character brought 

 out by the microscope is their decomposition. The oligoclase is much clouded an<l 

 altered, though a few crystals show clearly defined polysynthetic twinning. The 

 augite is almost entirely decomposed to calcite and chlorite, and the biotite of the 

 original diabase has all been changed to chlorite and iron. Calcite is found only in 

 very small scattered grains. The rock is very tough, fine grained, and holocrystal- 

 line." A few of the flakes and cobblestones show a laminated structure; they com- 

 prise less than 1 per cent of the whole. This character has not played an impor- 

 tant part in fracture except in this small proportion. 



