There are traces of sand, mixed with crystals of gypsum. A typical mix picked up from walking the city of Boston.
地板上还有不少混合着石膏结晶的沙粒，这是在波士顿步行不可避免会带上的痕迹。
2
Lichen fields-blooms of orange and gray over red sand and crusts of blackish gypsum-keep the soil stable and are a critical source of food for invertebrates.
Radon from building materials such as granite, brick sand, cement and gypsum and the like, in particular, contain elements of natural stone and easy to release radon.