THE f'HERTY LIMESTONE. 141 



initely known to be organic deposits. The origin of similar cherty deposits 

 in the Iron-bearing formation is discussed, in chapter v, section 2. What 

 is said tliere witli reference to the- " orig'inal rock" applies equally well here. 



It ajijjears that the chert of the limestone belt, whether original or 

 secondary, had in the main reached its jjresent condition before the accu- 

 luulaticjn of the immediately overlying Quartz-slate formation, since, as 

 already stated, verj^ numerous fragments of this chert are found included 

 within the slate at its base, and even in its middle and upper parts, and if 

 the concentration of this material in layers is due to secondary causes, there 

 must have been sufficient time between the deposition of the Cherty lime- 

 stone and the Quartz-slate to accomplish tliis. That the cliert has l)een 

 rearranged to a greater or less extent since its deposition, and that in the 

 cracks infiltrating solutions have brought additional silica, is more than 

 probable. At the time of this subsequent rearrangement and introduction 

 of silica, doubtless the tremolite was formed, although even this mineral 

 may have developed very early. In the formation of the tremolite, the 

 silica in solution had but to unite with a portion of the bases present, cal- 

 cium and magnesium. The origin of actinolite from an analogous rock, 

 except that it Ijore iron, is discussed in chapter v. What is tliere said 

 applies equally well io the tremolite in the Cherty limestone. 



Summary. — The cherts and limestones are placed together because as a 

 rule the chert occurs interstratified and thinly interlaminated with the lime- 

 stone. 



This member, instead of being continuous, often thins out and disap- 

 pears quite suddenly, so that it is mapped as occurring only where actually 

 found. It is j)rol)able that the Cherty limestone mend)er had a nuu-h 

 greater former continuity than at ])r(^s('nt. This is particul;n-lv probable 

 because fragments of it are aljundantly contained in the overlying Quartz- 

 slate mend)er. 



Its maxinnun thickness is 300 feet, and from this it varies io nothing. 



In petrogra])liic<al character the limestone is close to a dolomite which 

 is frequently tremolitic. 



