ROCK-WEATHERING IN CHURCHYARDS. 655 



respects we may regard the disintegration in towns as an exaggeration 

 of the normal rate. Still, the difference between town and country 

 may be less than might be supposed. Surfaces of stone are apt to get 

 begrimed with dust and smoke, and the crust of organic and inorganic 

 matter deposited upon them may in no small measure protect them 

 from the greater chemical activity of the more acid town rain. In 

 regard to the effect of daily or seasonal changes of temperature, on 

 the other hand, any difference between town and country may not 

 impossibly be on the side of the town. Owing, probably, to the influ- 

 ence of smoke in retarding radiation, thermometers placed in open 

 spaces in town commonly mark an extreme nocturnal temperature not 

 quite so low as those similarly placed in the suburbs, while they show 

 a maximum day temperature not quite so high. 



The illustrations of rock-weathering presented by fcity graveyards 

 are necessarily limited to the few kinds of rock employed for monu- 

 mental purposes. In this district the materials used are of three 

 kinds : 1. Calcareous, including marbles and limestones ; 2. Sand- 

 stones and flagstones ; 3. Granites. 



I. Calcakeous. With extremely rare exceptions, the calcareous 

 tombstones in our graveyards are constructed of ordinary white sac- 

 charoid Italian marble. I have also observed a pink Italian shell- 

 marble and a finely fossiliferous limestone containing fragments of 

 shells, foraminifera, etc. 



In a few cases the white marble has been employed by itself as 

 a monolith in the shape of an obelisk, urn, or other device ; but most 

 commonly it occurs in slabs which have been tightly fixed in a frame- 

 work of sandstone. These slabs, from less than one to fully two 

 inches thick, are generally placed vertically ; in one or two exam- 

 ples they have been inserted in large horizontal sandstone slabs or 

 " through-stanes." The form into which it has been cut and the 

 position in which it has been erected have had considerable influence 

 on the weathering of the stone. 



A specimen of the common white marble employed for monu- 

 mental purposes w'as obtained from one of the marble works of the 

 city, and examined microscopically. It presented the well-known 

 granular character of true saccharoid marble, consisting of rounded 

 granules of clear, transparent calcite, averaging about y^ inch in 

 diameter. Each granule has it own system of twin lamellations, and 

 not unfrequently gives interference colors. The fundamental rhom- 

 bohedral cleavage is everywhere well developed. Not a trace exists 

 of any amorphous granular matrix or base holding the crystalline 

 grains together. These seem molded into each other, but have evi- 

 dently no extraordinary cohesion. A small fragment placed in dilute 

 acid was entirely dissolved. There can be no doubt that this marble 

 must be very nearly pure carbonate of lime. 



The process of weathering in the case of this white marble pre- 



