14<^ NEW YORK STATE MUSEuM 



This is a limestone that in composition and structure at the 

 Rondout valley is apparently not very different in quality from the 

 Thirlmere rock. Analyses of the cement rock show less similarity 

 but observations indicate that it is also attacked. 



It is probable from all these facts that the shales and conglomer- 

 ates are better quaUty of wall than the limestones. 



A very acute observation along this line by Dr Thomas C. Brown 

 while employed on the staff of the Board of Water Supply is of 

 special interest. In studying local conditions he noticed that the 

 limestone blocks used in building the old Delaware and Hudson 

 (D. & H.) canal showed the effect of contact with the water. The 

 best place for measurable data seemed to be around the old locks 

 where squared and evenly trimmed blocks had been used. These 

 were, during the years of its use, from 1825 (approximately 35 to 

 40 years) subject to the action of water flowing or standing in 

 direct contact. The coigns of the locks, which were without doubt 

 freshly and well cut when laid, are now etched till the fossils and 

 other cherty constituents stand out from one eighth to one half 

 inch beyond the general block surface, and in some cases the pits 

 are an inch deep. That this etching is due to the water rather 

 than to exposure to weather is shown by the lack of such extensive 

 action on blocks used in houses and exposed a much longer time. 

 Blocks representing the Manlius and Coeymans were identified. 

 But there is no reasonable doubt that others would be similarly 

 affected. On some it would be less easily detected. 



On account of the disturbances another factor is introduced. 

 Rocks which readily heal their fractures are likely to furnish better 

 ground, i. e. more free from water circulation especially, than rocks 

 more brittle and slow to heal. Therefore in this district the shales 

 and slates such as the Hudson River series and the Esopus and 

 Hamilton shales are the best ground, while the Binnewater sand- 

 stone is the poorest. 



Cross sections. Probably in no region of like extent is it 

 possible to construct a geologic cross section of so many complex 

 features so accurately as can now be done of the Rondout valley 

 along the aqueduct line. The section is known or can be computed 

 to a total depth below the surface of 1000 feet, including 12 dis- 

 tinct formations, so closely that any bed or contact can be located 

 within a few feet at any point throughout a total distance of over 

 4 miles. 



The accompanying cross section contains as much of this data 

 as is now available [fig. 22']. 



