i^o.2.J SCUDDEE ON GEOLOGY OF FLORISSANT, COLORADO. 283 



the source of the volcanic ashes must have beeu close at hand seems 

 abundantly proved by the difference in the deposits at the extreme ends 

 of the lake, as will be shown in the sections to be given. Kot only does 

 the thickness of the different beds differ at the two i)oints, but it is dif- 

 ficult to bring them into anything beyond the most general concord- 

 ance. . 



There are still other proofs of disturbance. Around one of the gran- 

 itic islands in the southern lake basin the shales mentioned were cai^ped 

 by from one and a half to two and a half meters of sedimentary material, 

 reaching nearly to the crown of the hill, the lowest bed of which, a little 

 more than three decimeters thick, formed a regular horizontal stra- 

 tum of small volcanic pebbles and sand (A and B of Dr. Wadsworth's 

 note further on) ; while the part above is much coarser, resembling a 

 breccia, and is very unevenly bedded, pitching at every possible angle, 

 seamed, jointed, and weather-worn, curved and twisted, and inclosing 

 pockets of fine laminated shales, also of volcanic ash, in which a few 

 fossils are found (C of Dr. Wadsworth's note). These beds caj) the 

 series of regular and evenly stratified shales, and are perhaps synchro- 

 nous with the disturbance which tilted and emptied the basin. The 

 uppermost evenly bedded shales then formed the hard floor of the lake, 

 and these contorted beds the softer but hardening and therefore more 

 or less tenacious deposits on that floor. 



The excavation of the filled-up basin we must presume to be due to 

 the ordinary agencies of atmospheric erosion. The islands in the lower 

 lake take now as then the form of the granitic nucleus ; nearly all are 

 long and narrow, but their trend is in every direction, both across and 

 along the valley in which they rest. Great masses of the shales still 

 adhere equally on every side to the rocks against which they were 

 deposited, proving that time alone and no rude agency has degraded 

 the ancient floor of the lake. 



The shales in the southern basin dip to the north or northwest at an 

 angle of about two degrees, and an examination of the map will show that 

 the southern end of the ancient lake is now elevated nearly two hun- 

 dred and fifty metel'S above the extreme northwestern point. The 

 greater part of this present slope of the lake border will be found in the 

 southern half, where it cannot fail to strilce the observant eye upon the 

 spot, the southernmost margin, close to the summit of the divide, being 

 nearly two hundred meters higher than the margin next the school-house 

 hill. 



Our examination of the deposits of this lacustrine basin was principally 

 made in a small hill, from which perhaps the largest number of fossils 

 have been taken, lying just south of the house of Mr. Adam Hill, and 

 upon his ranch. Like the other ancient islets of this upland lake, it 

 now forms a mesa or flat-topped hill about ten or a dozen meters high, 

 perhaps a hundred meters long and twenty-five broad. Around its 

 eastern base are the famous petrified trees, huge, upright trunks, stand- 



