SAYLES: THE SQUANTUM TILLITE. 151 



" The exposures at this place are large blocks which show occasional 

 well marked, laminated strata of a fraction of an inch to two feet in 

 thickness. Since these beds all dip vertically and strike in the same 

 direction, I believe that they are practically in situ. Alternating 

 with these strata are layers that absolutely lack any evidence of water 

 sorting and water deposition. 



"The sorted layers, sometimes of uniform width for many feet, 

 consist of mudstone and sandstone. In the thickness of the entire 

 cliff section the}' may represent 15 or 20 percent. The unsorted 

 layers contain angular and irregularly shaped fragments of pinkish 

 granite (the species so common in the Roxbury conglomerate), gray 

 quartzite, greenish felsite, and dark green, chloritized melaphyre. 

 There are some rounded pebbles. These fragments and pebbles are 

 very variable in size. They range from grains of quartz and feldspar 

 T5^ to Ti5^ of an inch in diameter to large boulders, the largest seen being 

 four feet long. They are contained in a compact, greenish gray paste 

 or matrix which comprises 50 to 75 percent of the bulk of the rock. 

 Having no parallel structure of any sort — bedding or schistosity — 

 the paste breaks with an uneven fracture. Although the term 

 'tillite' is applicable to the whole section, I use it herewith reference 

 only to the unstratified, unsorted portions. ]My thin sections were 

 cut from a hand specimen of this tillite. 



" When examined with the microscope, the finer part of the rock is 

 found to be composed of minute grains of quartz and feldspar, very 

 small laths of sericite, and a highly refracting, granular substance, 

 uniformly distributed, which is probably epidote. The quartz and 

 feldspar are so fine that little can be distinguished. The sericite laths 

 show a tendency to parallel orientation, thus indicating some shearing 

 in the rock, but not enough to produce a visible schistosity in the hand 

 specimen. The laths are small and of nearly uniform dimensions. 

 The paste may be said to consist of particles having the same size as 

 these laths, or smaller. This mica constitutes between 20 % and 

 25 % of the matrix. 



" In the paste are scattered grains of quartz and feldspar and small 

 fragments of granite, quartzite, and melaphyre, as seen in the hand 

 specimen. These grains and fragments are usually angular. The 

 larger quartz particles exhibit slight wavy extinction and some crack- 

 ing, and also fine peripheral granulation accompanied by the marginal 

 insertion of sericite laths, characteristic of the early stages of dynamic 

 metamorphism. Of the feldspar grains, examples of orthoclase, micro- 

 cline, microperthite, and plagioclase (albite to oligoclase) were de- 



